Continuous casting is a well known method for converting liquid metals into solid materials of constant cross-section in continuous or semi-continuous lengths, convenient for use as cast or suitable to further forming by well known metal working procedures.
Equipment for continuous casting is well known in the metals industry. Several schemes are in common usage. For the casting of relatively low melting metals and alloys, (below a melting point of about 1200.degree. C.) a type of system using solid crucibles and dies is often used.
In this system the solid crucible is usually constructed of graphite. The metal may be introduced as solid alloy or components and melted directly in the crucible, or may be added as molten metal from an exterior melting and/or alloying source.
The crucible is generally maintained at some temperature above the metal melting point via externally applied heat, as by induction, electrical resistance heaters, gas flames, or other means well known in the industry, and is connected with an orifice or feed section that allows molten metal to flow into a casting die. The die is most often machined from graphite, and is held tightly in contact with, inserted in, or in some fashion attached to the orifice or feed section so as to form a leak proof seal.
The die can be placed at an angle, generally 90.degree., to the crucible, in which case the method of casting is referred to as "horizontal" casting, or may be placed below the crucible, in which case the method of casting is referred to as "vertical" casting.
Molten metal feeds by gravity and/or pressure through the feed section and freezes to solid metal at some point. Freezing is controlled by cooling devices such as water cooled plate coolers, attached or pressed against the outer surface of the die or adjustable water or gas cooling R.W. 7 July 1987 by 15 July 87 probes, disposed in the die. Casting is accomplished in a continuous manner by withdrawing the solidified metal via some withdrawal mechanism.
Silver, gold, copper, and aluminum and their alloys are commonly produced by both horizontal and vertical casting, using graphite dies and crucibles. A die used in such a furnace is shown in U.S. Pat. No. 4,295,516, the furnace being of a small size suitable for installations in existing facilities.
It is desirable to produce other alloys by the continuous casting process, most particularly, for instance, alloys based on nickel (Ni), nickel-chromium (NiCr), palladium (Pd), platinum (Pt), iron (Fe) and cobalt (Co). However, it is not possible to use a graphite die and particularly graphite crucible systems for these alloys, as they all readily dissolve carbon (graphite). This dissolution not only changes the properties of the metals; it erodes the crucible and die, rendering them almost immediately unusable and ultimately resulting in metal leaking through the system, producing a partial or total metal loss into the heating portion of the melting system.
U.S. Pat. No. 4,175,611 discloses plasma coating graphite dies with various materials for use when casting the above materials, which various materials may improve the wearing characteristics of the die.